My apologies if this has been covered in the last gazillion posts...but do we know if he generally parked his work truck in the garage? If not, I am really curious if the neighbor's camera caught what time he pulled it into the garage...
Because I can't help but think he might have lost it getting the girls ready for bed. They were probably wired from the party. Maybe they wanted to wait up for mommy. Meanwhile he's anxious for time to himself to sext his AP or look at some *advertiser censored*. And he loses it with one of them. Maybe her sister witnesses it...The beds were unmade, but maybe he didn't make them while SW was away.
Are they already loaded in his truck long before SW gets home? During these hours, he's figured out where he wants to hide the bodies, planning ahead on how he is going to kill his wife, she'll never forgive him for what he's done. And anyway, he'd rather be single. Maybe fantasizing about his life as a single man. And she's so late it's messing up his plans.
Did he wait in the bedroom? She kicks off her shoes, puts her purse on the counter and sits on the couch to look at her phone. She's tired but not ready for bed. He calls down, are you coming to bed?
She peeks in the girls room on her way, sees them missing, but thinks maybe they're in the parents bed since they like to sleep there. She walks in, no girls. She asks him, where are the kids? He starts his story, their "emotional conversation". And he kills her.
The rest of his plans to have them 'disappear' are rushed because of her flight delay. And NU (thankfully!!) derailed the rest.